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#51 Asbytec

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Posted 14 January 2014 - 09:51 PM

" The realitive effects from aperture and aberrations on such image will be realistic, but its not what you would encounter at the eyepiece."

Its not clear what they mean by this, do they mean we will not see the diffraction effects as shown? Or do they mean don't expect to see the pattern so clearly due to seeing? I will admit I've had a hard time getting Aberrator to put up patterns that are exactly what I see at the eyepiece, so the latter may be true.

But, as an approximation, it does pretty well with a scaled very good image of Ganymede when applied to an aberrant and obstructed 150mm aperture. Maybe not perfect, but pretty darn close, actually. So, I trust it as far as I can throw it. The image above might not be perfect, but it has characteristics that make it look pretty darn close including the grey floor of a 1" arc craterlet when applied to a 150mm aperture. That is consistent both with my experience and from what I can gather from theory.

#52 David Knisely

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Posted 14 January 2014 - 11:26 PM

Asbytec posted:

Its not clear what they mean by this


Yes, it is quite clear. The relative effects of central obstruction or aberrations on an image are fairly accurate, but the view will not match that of a given aperture on an actual planetary object. You put in an image of the given object and *not* the actual view of the object itself in that given aperture. In other words, it demonstrates the effect of the degradation caused by obstructions or aberrations on a given image, but does not precisely duplicate what is seen with the actual view with the *real* object. That would require an accurate lower-contrast image that matches what could be obtained with perfect optics and the human eye. This isn't what the program uses (and the limit of 10 pixels per arc second really kills the ability to simulate smaller features).

But, as an approximation, it does pretty well with a scaled very good image of Ganymede when applied to an aberrant and obstructed 150mm aperture.


Huh?? Are you saying you have actually seen detail on Ganymede in a 150mm MCT?? Ganymede in a 150mm aperture is basically a featureless disk less than 1.84 arc seconds across. This is *especially* true of a 150mm scope with more than a 30% obstruction (Ganymede's angular diameter is only roughly the diameter of the diffraction disk of a star in a 150mm aperture and not much larger than a bright star's spurious disk). Even in my 9.25 inch SCT near opposition, I can only get the vaguest hint of a bit of darkening in one small area on Ganymede at over 400x. My 10 inch (21% central obstruction, 1/19th wave p-v wavefront primary) does slightly better on showing a vague darker area on that moon, but that feature is of *very* low contrast. Otherwise, there is very little obvious detail on Ganymede in that aperture. In my 14 inch Newtonian (22.5% central obstruction) I can see a little more, but the shadings (when they are visible) are still quite subtle. They have far lower contrast than the heavily stacked and processed CCD images of that moon taken by Damian Peach in his 14 inch telescope. Aberrator is a useful piece of software, but not for simulating what the views of real objects or small scale lunar features will look like precisely when viewed by the eye through the telescope. Clear skies to you.

#53 Asbytec

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Posted 15 January 2014 - 05:47 AM

Huh?? Are you saying you have actually seen detail on Ganymede in a 150mm MCT??

One of the reasons I am a believer in theory is having seen it working as advertised. But, absolutely a 150mm can resolve features on Ganymede.

I guess it depends on what you mean by resolved, though, and what features are observed. Nothing is perfectly clear and distinct like an image, but you can see Osiris and Phrygia Suclus as brighter specks dancing near the limb easily enough. Any darker features are just that, maybe a bit if very indistinct darkening approaching the limb and nothing more. No riles, no patches, nothing other than a darker hemisphere than the other. I will say, however, that the dark feature Perrine Regio was totally not seen even though it looks possible. As you say, it apparently has not enough contrast. So, outside of a darker hemisphere, I doubt 'real' dark feature resolution can be done easily.

So, dark features maybe, but bright craters are definitely seen on Ganymede.

You're description of seeing with 9 and 10" aperture sounds reasonable and more like a bit higher resolution of those apertures. I can only credit this to excellent tropical seeing conditions, but the brighter craters are there to be seen and maybe, just maybe, a slight hint of darkening. Eddgie can see darker features more clearly in his C14, to me its just a very weak darkening to some point on the disc. I doubt it's as good as your 9 and 10" scope show it, but it is detectable.

http://www.cloudynig...6297950/page...

Io has some distinctness about it, too, but nothing "resolved." Those are two Jovian moons a 6" can show as something other than a disc.

Yes, apparently you can resolve detail on an object the diameter of the Airy disc provided, as you say, contrast is large enough (or maybe expansive enough, too.) More to the point, though, the same resolution behavior should exhibit itself on the moon, even though we're not dealing with a single Airy disc (Ganymede might be more than one Airy disc, too, but it's at least an expanded PSF with peaks of varying brightness 4x larger than an optical point source.)

Yea, please don't misunderstand. I agree with you Aberrator is probably not perfect working with extended objects. But it's amazingly close. Here's one of Ganymede with an aberrant 6" applied. Not bad, even though the dark Regio Galileo is better resolved in this image than in the real world, Osiris is plainly visible.

I'd expect similar results with craterlets on Plato's floor. In fact, I am hoping folks report such findings.

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#54 David Knisely

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Posted 15 January 2014 - 01:07 PM

Asbytec wrote:

One of the reasons I am a believer in theory is having seen it working as advertised. But, absolutely a 150mm can resolve features on Ganymede.


I am sorry, but a 150mm aperture cannot resolve *any* features on Ganymede. It is utterly impossible, as the diffraction effects in a 150mm aperture will totally obscure *any* detail on that moon (especially with a whopping 31% central obstruction making things even worse). The diffraction disk of a star in a 150mm aperture and the disk of Ganymede are just too similar in size to allow anything to be even remotely detected on that moon's tiny disk (see diagram below). The telescope simply isn't large enough to pick out any real detail on Ganymede. In fact, at only a 150mm aperture, the aperture is just barely large enough to make the disk of Ganymede itself become resolvable rather than just blurring into a spurious disk in the diffraction pattern of a star-like object. Any detail you might seem to see on Ganymede in a 150mm aperture is totally illusionary, possibly caused by the "noise" of the eye/brain system (or perhaps some seeing effects). It isn't real detail.

I'm afraid that if you believe you see detail on Ganymede with such a limited aperture, this might make me wonder just a little bit about your alleged sightings of the e craterlet in Plato. Clear skies to you.

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#55 azure1961p

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Posted 15 January 2014 - 04:14 PM

Not true actually Dave. SKY & TELESCOPE some years back had an article about Gary Nowak seeing a polar brightening with a 6" Apo . I duplicated that observation with my 8". The illustration is accurate you portray but the airy disc size comparative isnt representing contrast resolution that can be seen smaller than its radi or diameter. A stellar diffraction pattern is a different matter altogether.

I think you are misunderstanding resolution Dave.

Pete

#56 brianb11213

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Posted 15 January 2014 - 05:29 PM

There appears to be a deficiency in David's diagram: in practice there is a significant brightness gradient in the inner region of the Airy disc, and consequently it doesn't seem unreasonable to be able to resolve high contrast features which are around half the diameter of the Airy disc. I prefer to think of the extended object resolution threshold of an instrument as about the Rayleigh limit (5.45 arc sec divided by the objective diameter in inches). The presence of a central obstruction of the size typical of reflecting or compound instruments of the type likely to be used for lunar/planetary observation really doesn't make much difference, the contraction of the Airy disc with increasing CO being offset by contrast loss.

Anyhow the highest contrast "feature" there is to see on any of the Galilean satellites of Jupiter is the contrast between lit and unlit portions during the beginning or end of an eclipse. At mid eclipse the satellite should show a perfect half phase. Experience is that this is seldom if ever seen with 6" of aperture ...

#57 David Knisely

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Posted 15 January 2014 - 07:02 PM

Not true actually Dave. SKY & TELESCOPE some years back had an article about Gary Nowak seeing a polar brightening with a 6" Apo . I duplicated that observation with my 8". The illustration is accurate you portray but the airy disc size comparative isnt representing contrast resolution that can be seen smaller than its radi or diameter. A stellar diffraction pattern is a different matter altogether.

I think you are misunderstanding resolution Dave.

Pete


No, I am not misunderstanding anything. We are talking about "detection" of detail. People claim to see all sorts of things right at the limits of visual observation (even in Sky and Telescope), but in this case, the physics of the situation kind of trumps the observation. I do fully understand resolution (I have a B.S. in Physics/astronomy). Detection of very low contrast non-linear detail that is significantly smaller than the stellar diffraction disk diameter for a given telescope is just not possible because the diffraction effects that create the pattern tend to obscure that detail. Reliable claims of observations of detail on Ganymede were first done with apertures larger than six inches (example: observations by H. Camichel, N. Lyot, and M. Gentil, using a 15.2 inch (38 cm) refractor on Pic-du-Midi in 1941). Claims of observations of detail on Ganymede in a 5.9 inch Mak-Cassegrain with a whopping 31% central obstruction are very highly questionable to say the least. It also isn't a question of pure resolution either. The magnification needed to get a 1.8 arc second object up to something even half the apparent size of the full moon would be 500x, which would yield a very dim image of that object in only a 5.9 inch telescope. That aperture would have to approach 50% larger (say, something larger than eight inches) for such an observational claim to be even slightly credible.

As for my diagram, for demonstration purposes, it is reasonably close to being fully correct. The central bright disk seen with stars at high power is *not* the "diffraction disk" (and some authors don't even call it the "Airy" disk either). The diameter of the diffraction disk is defined as the diameter of the central diffraction pattern *at the first minimum* of the pattern (the first minimum is the very center of the first dark ring out from the bright central "spurious" disk). For an unobstructed 150mm aperture in visible light, this is 1.845 arc seconds (twice the Rayleigh Criterion figure of just under 0.923 arc seconds). For a 31% obstruction, the first minimum's diameter is slightly smaller at about 1.677 arc seconds, but as can be seen by closely examining the diagram I created pixel by pixel, the 1.8 arc second drawn disk of Ganymede (72 pixels wide) is still just slightly larger than the first minimum of the adjacent star diffraction pattern (67 pixels wide). The largest Ganymede ever gets is around 1.8 arc seconds and right now near the current opposition, it is 1.72 arc seconds across. This is so close to the size of the diffraction disk (and not all that much larger than the bright central "spurious" disk of a star's diffraction pattern) that the effects of the way light is forming that pattern will obscure detail significantly smaller than that diffraction disk's size. Sorry, I just don't buy claims of detail being visible on Ganymede in modest apertures like a mere 150mm. Clear skies to you.

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#58 Asbytec

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Posted 15 January 2014 - 08:02 PM

I would think the spurious disc (not the Airy disc) would be the smallest resolvable feature possible, yet we can see objects 4x smaller than the Airy disc.

To begin with, the spurious disc is already about half the diameter, so you're halfway there, already. In point source resolution without a black space, there is plenty of empirical evidence suggesting very tight double stars can be 'resolved' down to 0.5 times the Raleigh limit - 1/4th the Airy disc diameter. (My personal best is 0.62 times Raleigh limit no doubt influenced by the presence of an obstruction, as is the case with 7 Tau easily split with sufficient dark space to indicate something smaller than Dawes is possible.)

At this spacial frequency, there is not sufficient contrast between to brightly lit spurious discs to show a dark space for hard resolution. And we're well beyond Dawes at this point, anyway. This is a different animal than an extended object, but the example shows the Airy disc is not the limiting feature we think it is.

As an extended object begins to exceed 1/4th the Airy disc diameter (the working definition of an optical point source), it's PSF begins to expand noticeably. When the disc radius is equal to Lambda/F it's FWHM (which is an approximation of what we see near Dawes) is much larger than the PSF of the point source Airy disc at FWHM by a factor of two. In other words, Ganymede is twice the diameter of the spurious disc leaving plenty of room for a high (enough) contrast feature to offer an Airy disc of it's own superimposed on the twice as large FWHM of Ganymede.

It's in the form of a gradient, as Brian says. The high contrast bright feature can peak above the surface intensity of Ganymede, as I understand it. And if the peak is high enough (contrast), such a bright object can be seen on the expanded PSF on an object of Airy disc diameter (whose PSF is twice FWHM of a point source Airy disc.) A dark object, too, if it's of sufficient contrast. Otherwise it might appear as an intensity fall off on one hemisphere if the feature is large enough. Galileo Regio is pretty large and seen as a less bright hemisphere (or limb shading), Perrine Regio is not large and was not seen.

If you have ever seen the diffraction rings around Jovian moons, you will note Io and Europa are more star-like in appearance. Ganymede's rings are more washed out indicating it is, indeed, not a point source and therefore does not offer a point source PSF. Io and Europa are at a diameter that is roughly 1" arc, or about half the Airy disc diameter, and their PSF is barely enlarged beyond that of a point source.

One might even resolve a very high contrast feature on either Io or Europa if one existed on their surface. The resolution would be very difficult and very similar to a very tight equally bright double with a separation near half the Raleigh limit. Io does appear elongated and this may be the cause. Resolution on Io? Surely I jest (:lol:) I dunno, maybe. Depends on the definition of resolution. Maybe from the behavior of its PSF we can say we resolved it's brighter equator from its darker poles even though we cannot see what's actually going on. Nothing is actually 'split', it's simply elongated.

That's theory and accords with my experience with two bright crater 'specks' seen on Ganymede's surface when seeing is at least diffraction limited. Any induced aberration makes detection that much more difficult.

This is high resolution applicable to lunar observing where contrasts are very high. You are correct, the obstruction makes a tiny difference of about 10% in the realm of the very tiny (near the Airy disc and inside the first ring.) That is a difference between resolving a crater that subtends 1" arc and one that subtends 0.9" arc (which turns out to be a the difference between crater about 1.1 mile in diameter and one that is 1 mile in diameter at the lunar mean distance.) It minor, but doable when your scope is operating in near lab like conditions in the real world. I have seen both "e" on Plato, and IIRC, one closer to 1 mile elsewhere, and surface high (enough) contrast features on Ganymede.

Edit: I was upwards of 400x on Ganymede. Of course, there is no further resolution to be had, only image scale and brightness as you say. But, the relative contrast should remain unchanged and the dimming does have (unknown to me) physiological effects. Being up that high didn't seem to hurt anything, it was just easier to look at. I'd assert the presence of an obstruction was helpful, if minor. It was minor enough.

Anyway, it's a great discussion and I think it applies directly to the high resolution needed for small Plato craterlets.

#59 Asbytec

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Posted 15 January 2014 - 08:31 PM

...and consequently it doesn't seem unreasonable to be able to resolve high contrast features which are around half the diameter of the Airy disc.

Brian, I agree Raleigh is in the ball park for resolution of features with sufficient contrast. Certainly 100% contrast with about 28% remaining contrast transferred. It is likely more difficult on the moon itself and Plato in particular cine object contrast is lower. This threatens Raleigh limit as a limit for extended objects, but if two points leave 28% contrast and it appears black then lunar features of that angular separation can leave something less than 28% final transferred contrast and appear grey.

Yes, the first bright ring (mostly) does seem to make any high contrast features, such as a crater floor, much more grey than black provided it is of a dimension that is lies under those rings in a very complex way. As long as that grey is a different level than it's surroundings, maybe by about 5% in accord with Dawes, then that crater floor should be seen if it is large enough to separate the brighter (and infinite number of) spurious discs in the vicinity.

On your quote above, actually I'd think you could see anything smaller than 1/2 the Airy disc diameter for bright sources all the way down to a geometrical point provided it is bright enough. It will form it's own spurious disc that will be about half the diameter of the Airy disc. Imagine being able to 'resolve' geometrical points on the lunar surface or Jupiter, for that matter. Now that's some resolution when you can 'resolve' points on an extended object. :grin:

But we should see them in the same way we see stars - as spurious discs at about FWHM when the extended object disc is at least FWHM or larger - if transferred contrast is sufficient, as you say. On the moon, this could affect the detection of crater specks. We might not know how small they are, we'd only know how bright or how much contrast they offer. One might imagine they'd have to be of some diameter and albedo to reflect enough light putting up enough final contrast.

Fascinating discussion...the proof lay on Plato's floor.

#60 azure1961p

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Posted 15 January 2014 - 10:19 PM

I would still disagree (funny because Im not the one making the observation with 150mm but I haven't tried either) .

Buddy, seeing a half a Ganymede , or rather a crescent of Ganymede via an eipse by another Galilean moon was actually had with a 4" apo as reported on thes boards by Buddy Barby. It goes back a couple years but I recall his reporting that quite clearly. And mind you, he wasn't seeking it out as a threshold achievement infact he didn't expect to see such a thing but alas, he reports he had.

David, The trouble that's muddying thes waters is that a stellar diffraction pattern is created by a virtual point source. Ther can be no such contrasts - even if there were to be had from such a minuscule point - hence the pokerface diffraction pattern. By contrast (literally) the surface area of a galillean moon is a different dynamic altogether. Its not a virtual point source at all and the area subtended (granted beyond the angular res if the aperture) is producing a sizable surface area light at any point .

Take Io for example. Before there was a CN with an imaging forum packed with egg shaped Io images, Pickering saw it as ovular with a 5" aperture - not even a 150mm. By using the eclipsed Ganymede as an example though Brian you could say that Pickering couldn't possibly detect the polar darkening evidenced by the ovular Io he observed. Of course he didnt realize it was albedo effect at the poles he assumed it was egg shaped in fact. The truth of course is Pickering with the 5" refractor was RESOLVING the lesser albedo of the poles (however crudely) - and on a disc virtually 1" across. A far greater feat than seeing a half a Ganymede in semi-eclipse.

The trouble here is the moons aren't stars and while diffraction effects certainly serve to mask contrasts it doesn't obliterate them to the degree of absolute erasure. Pickerings Io observation is strong evidence of that this is the case . The fact that Io's poles are hardly black like that of an semi eclipsed Ganymede and it underscores the point. Merely darker material was enough to change the apparent shape of the object. When you factor in the understanding the brighter equator of Io is only about 0.3 to 0.5 arc second this should have been completely missed in such a hole aperture.

I'm bypassing Norme here and going for Pickerings example as its from a time when no high res images were available of course and so that observer couldnt be fooled by prior suggestion.

I respect both of you but Ive got to disagree .


Pete

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#61 David Knisely

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Posted 16 January 2014 - 12:25 AM

azure1961p wrote:

David, The trouble that's muddying thes waters is that a stellar diffraction pattern is created by a virtual point source. Ther can be no such contrasts - even if there were to be had from such a minuscule point - hence the pokerface diffraction pattern. By contrast (literally) the surface area of a galillean moon is a different dynamic altogether. Its not a virtual point source at all and the area subtended (granted beyond the angular res if the aperture) is producing a sizable surface area light at any point .

Take Io for example. Before there was a CN with an imaging forum packed with egg shaped Io images, Pickering saw it as ovular with a 5" aperture - not even a 150mm. By using the eclipsed Ganymede as an example though Brian you could say that Pickering couldn't possibly detect the polar darkening evidenced by the ovular Io he observed. Of course he didnt realize it was albedo effect at the poles he assumed it was egg shaped in fact. The truth of course is Pickering with the 5" refractor was RESOLVING the lesser albedo of the poles (however crudely) - and on a disc virtually 1" across. A far greater feat than seeing a half a Ganymede in semi-eclipse.


With the eclipse, you have a very high contrast large-scale feature (the shadow of another Galilean moon) which is a good percentage of the diameter of the moon itself. That is not the case with finer albedo features on the Galilean moon's surface. The times when I have seen even a hint of albedo features on Ganymede, it was using my 9.25 inch and 10 inch apertures. They were hardly obvious in those apertures (pretty darn marginal in the 9.25 inch SCT actually), so I really have a something of a problem believing that much if anything of them can be seen in a 150mm aperture (especially one with such a large central obstruction). Probably the best view I ever got of the Galilean detail was during a transit of Ganymede across Jupiter, as then, you could see both the color and variable intensity of that moon's disk. Against the black background of space, it wasn't nearly as obvious. Shadows or large-scale high-contrast albedo features are one thing, but Ganymede's surface markings are somewhat different.

It would be a lot easier to get back to talking about craterlets on the moon, where the contrast is frequently fairly high even at some physically small scales. Clear skies to you.

#62 Asbytec

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Posted 16 January 2014 - 08:49 AM

Pete, you can bypass me all you want. :grin:

Retracing Pickering's observations was an enormous treat, a taste of history repeated. You were there as was Jason and Edggie, et al. I thank Jason Berry for bring the subject up, something I never thought possible. But it is, Just as Pickering claimed. He may not be a nut case, after all. Maybe I am. :grin:

I respect everyone who's participated in this discussion and have a deep appreciation for David's lasting and meaningful continuations. And he's right, this is a topic of Plato's craters. We should be discussing that. Barring that, however, we can talk about resolution for a bit because it pertains to Plato while we wait for observations to trickle in.

Pete, you are so right, though. Pickering did not know what he was looking at. We do, but that makes repeating his observation no less exciting. And diffraction does hamper observation, we just have to know at what point this is so.

As I've said before, with emphasis, this is a great discussion and I look forward to reports, once again, from Plato. Not only is it exciting, but we can learn a lot about ourselves and our equipment. And theory and the real world.

#63 Asbytec

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Posted 16 January 2014 - 10:36 PM

Looks like "JAN 22 2214 SETTING" is the next opportunity to observe Plato. It's late in the evening, so maybe seeing will improve somewhat.

Maybe at this sun angle, enough of the craterlet floor will be dark allowing resolution, too. At higher sun angles it would appear that contrasting darker shadow would be minimal and probably fall below Raleigh limit for the smallest craters.

David, above you refer to the CO as a brighter first ring effect. That's correct. Its interesting I think of the CO affects in terms of smaller spurious disc. Both are minimal, and maybe the smaller disc is more minimal. Both can have an effect of offering less contrast on the crater floor but allowing us to resolve slightly smaller until that crater floor is lost in the balance of both effects.

By the way, don't forget Plato's albedo. It can be quite challenging, too. Below is my visual impression and cannot swear it matches images very well.

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#64 Asbytec

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Posted 17 January 2014 - 01:57 AM

Back to Ganymede and the topic of resolution as it may relate to Plato for a moment. Here is an image of Ganymede taken with an 8" SCT. It's not a clear image, but it tells me the contrast is on the focal plane. Otherwise the camera would not have been able to detect and enhance it. And if enough contrast is on the focal plane, it can be seen by the human eye, too, though clearly not nearly as well as the image.

Also, note the level of resolution available with an Airy disc ~1.4" arc in diameter (or maybe closer to 1.2" arc obstructed) on a disc ~1.8" arc in diameter. Clearly resolution below the level of the Airy disc (smaller by ~0.4" arc than a 6" aperture) is already well underway on an a extended object not much larger than the Airy disc itself. This suggests resolution is also occurring within a 6" Airy disc, albeit more weakly.

Below is an Aberrator simulation of Ganymede through an 8" aperture. It probably resembles, though not perfectly, closely the human eye response than a processed image. Maybe some processing of this image might reveal something similar to the image above, if the image below contains the dynamic range necessary.

But, this just shows what smaller apertures can do near the Raleigh limit. I expect similar results on Plato if anyone reports them in the coming weeks.

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#65 edosaurusrex

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Posted 17 January 2014 - 06:52 PM

The sunset time is when the Sun's altitude will be 12 degrees at Plato's center and getting lower. Start looking for the craterlets before the sunset dates.

#66 Asbytec

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Posted 17 January 2014 - 08:51 PM

Okay, a couple of nights prior, maybe tomorrow. If the weather will ever clear, again, within the span of a human life time.

#67 azure1961p

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Posted 17 January 2014 - 11:10 PM

The sim looks close - but my observations are too long ago when I had good seeing 9-Puckering or better . I do recall with utter certainty on of the poles appearing like it had a polar cap - about 25-30 percent of the disc from the pole end - somewhat like a Martian polar good. That particular night ( being the best ) I had the serendipity of seeing the exact same image the next day online the following morning by the owner if excelsior optics - Mauro D. He photographed more than I saw with my eye (with a ten inch newt) but the polar brightening was spot on. Other times I had seen vague shadings - without the saturday morning serendipity if a follow up photo ( this is prior to CCDs catching on as big as now) The image you link to gives a nice feel for the size of the details that are visible , though a darn sight more pale! With some advice from on of the experts he did less processing and farther down the page his image is clearer. I don't think Im getting 9 Pickering anytime soon this winter so it may be a while before I can duplicate those sightings. An interesting topic nonetheless.

Other observers who've seen details on Ganymede with medium apertures is Carlos Hernandez with his 9" Mak and David Gray with a 10" f/7 newt.
Both were able to actually see fine streak like details . I'm happy with the polar hood look!

Pete

#68 OrlandoMatt

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Posted 24 January 2014 - 04:41 PM

wait wait wait, it has mini craters?!

I need to go setup, bbl

#69 Asbytec

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Posted 24 January 2014 - 10:48 PM

wait wait wait, it has mini craters?!

I need to go setup, bbl


:lol:

Report back. I missed my chance on the 22nd. But, I guess the mini craters are still there. Dunno, haven't seen them in a while.

#70 azure1961p

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Posted 25 January 2014 - 11:59 PM

wait wait wait, it has mini craters?!

I need to go setup, bbl


Sure does but boy they are not easy.

Pete

#71 OrlandoMatt

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Posted 27 January 2014 - 01:48 PM

Being new to the moon and remembering it being the first thing I saw as a kid; Plato is easily one of my favorite objects! It's just so round! Anyways I'm really going to investigate it after all the info in this thread. I'll report back soon!

#72 azure1961p

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Posted 27 January 2014 - 10:13 PM

Plato is one of those great potential areas for me that I consistently miss best lighting for. Either its way too dark and shadow cast or the lighting more often then not at my observing hour, is so steep its pointless. Its just off in longitude enough my schedule rarely allows for a great lighting opportunity when I can observe it. The challenges are there and craters all the way down to invisibly small but I'm usually witness to the bright four or five.

Pete

#73 photonovore

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Posted 28 January 2014 - 02:38 AM

Set up your 6" scope in a field and set up a target board. Have an assistant put up alternate sheets of white paper with one of the following symbols(each equal diameter)upon each printed in black-- show to the observer in random order and multiple times (including intermittently repeating the same figure)--and sized/at a distance which equals ~1-1.5"arc apparent angular dimension:

c e a o @ *

or make up some little disc graphics with "bites" out of cardinal parts of the circumference with a crescent or two thrown in for good measure.

Attempt to identify each individual symbol as it is presented and compile the results and correlate.

You will end up with an answer as to what you can and cannot resolve with that telescope at such an angular dimension--this with nearly perfect "seeing" and with near perfect contrast. If you only make a hash of consistently identifying the various figures in this exercise you can rest assured that anything in the sky of similar angular size within which you *think* you are seeing detail... you certainly are not.

#74 azure1961p

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Posted 28 January 2014 - 08:12 AM

A way to condense or shorten the distance if this set up is to use a reflective ball like a silver Christmas bauble with the reflection of the words within it. By breaking the baubles size down by degrees and arc seconds per its diameter you can get the size of the type or letters, fonts, whatever. If say the larger but farther the symbols are from the bauble the better so as not to get fisheye distortions. I mention this because doing it another way can be really hazardous to the image with ground thermals boiling away the details . The bauble closes this distance for a cassegrain of 6" to under 100 yards . You could focus nearer but then the mirror spacing for near focus introduces SA aberrations.

Pete

#75 Asbytec

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Posted 28 January 2014 - 09:29 AM

If you only make a hash of consistently identifying the various figures in this exercise you can rest assured that anything in the sky of similar angular size within which you *think* you are seeing detail... you certainly are not.


It's an interesting test, in near lab like conditions, if someone wants to conduct it. It'll tell you what you should be seeing if real world seeing is diffraction limited. I might be interested to perform it to know what is possible in the "lab", rather than what is probable in the real world. When seeing permits, what is probable comes pretty close to what is possible. If 1", for example, is well within what is probable, then something smaller might be possible.

The moon, actually, is such a target at a known distance with 1" and smaller features on it. When seeing is diffraction limited one can discern, minus the ambiguity of various and random characters, familiar features such as craterlet floors. There may be some level of "thinking" we see it, but experience will be a good guide as to whether that fluttering dark spot is a crater floor or not. Then you can say, "you certainly do."

As a hypothesis, experience tells me I could easily discern o and c at 1" arc. Letters a and e will likely be grayed out, and might be able to see the larger space in the lower half of the a. There's probably no chance of resolving the more narrow space in the upper e, but the e might be discerned as such, anyway. The ampersand might show a white feature in it's center, but I doubt the double arcing lines will be resolved. The asterisk will likely just be a dark spot. It would be interesting to note whether the asterisk's spikes can be resolved.

Successfully and knowingly observing sub arc second features on both Jupiter and Ganymede in diffraction limited seeing have given me the experience to know about what my scope and acuity can do. A similar test with pin points simulating double stars might be interesting, though. Maybe this rainy season, right now Jupiter awaits on the zenith and I await the last of the day's clouds to vanish.

Anyway, the thing is, the difference between thinking we saw something and knowing we saw something just come with experience and personal integrity. If I resolved the space in the upper e or the spikes on the asterisk, would anyone believe it? Not until they tried it themselves, and probably dependent on whether they succeeded or failed. :lol:






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